This invention relates to electric power supplies, and particularly to a switching power supply capable of a.c. to d.c. voltage conversion, featuring provisions for attainment of closer approximation of the input current waveform to a sinusoidal wave, and a higher power factor, than by the comparable prior art.
A conversion from an alternating to a direct current is possible by a rectifying and smoothing circuit comprising a rectifying circuit having a diode connected to an a.c. power supply, and a smoothing capacitor connected to the rectifying circuit. This type of rectifying and smoothing circuit possesses the disadvantage, however, of a somewhat poor power factor as a result of the fact that the smoothing capacitor is charged only at or adjacent the peaks of the a.c. voltage of sinusoidal waveform. Another drawback is that it is incapable of adjustably varying the d.c. output voltage.
Japanese Unexamined Patent Publication No. 8-154379 represents an improvement of the rectifying and smoothing circuit above. It teaches a switching power supply comprising a rectifying circuit, a smoothing capacitor, a d.c.-to-d.c. converter circuit, and an inductive reactor for a higher power factor. The reactor is electrically connected between the pair of output terminals of the rectifying circuit upon closure of a switch included in the d.c.-to-d.c. converter circuit. The desired improvement in power factor is thus attained, as the current flowing through the reactor varies in amplitude in step with the a.c. input voltage.
This prior art switching power supply has proved to have its own shortcomings. There flows through the reactor the current not only for power factor improvement but for charging the smoothing capacitor. The reactor had to be inconveniently bulky for carrying such large current and actually gave rise to power loss to an intolerable degree.
The present invention seeks to improve the switching power supply of the noted type having an inductor, for still higher efficiency without impairment of its inherent advantages.
Briefly, the invention may be summarized as a switching power supply capable of translating a.c. voltage into d.c. voltage. Included are a rectifier circuit connected to a pair of a.c. input terminals for rectifying a.c. input voltage, a transformer having a primary winding, a rectifying and smoothing circuit connected between the transformer and a pair of d.c. output terminals for providing d.c. output voltage. The rectifier circuit has a first output conductor for outputting a first rectifier output voltage, a second output conductor for outputting a second rectifier output voltage which is substantially the same as the first rectifier output voltage, and a third output conductor or ground-potential output conductor. Also included are a switch connected between the transformer and the third output conductor of the rectifier circuit in order to be turned on and off at a repetition frequency higher than the frequency of the a.c. input voltage in order to keep the d.c. output voltage at a prescribed value, an inductor connected between the first output conductor and the third output conductor of the rectifier circuit at least via the switch, and a smoothing capacitor connected between the first output conductor and the third output conductor of the rectifier circuit via the transformer and the inductor. Additionally, and perhaps most characteristically, an ancillary charging circuit is provided which has an ancillary winding which is electromagnetically coupled with the primary winding of the transformer and which is connected between the second output conductor of the rectifier circuit and the smoothing capacitor.
Utilizing the voltage across the ancillary winding added to the preexisting transformer, the ancillary charging circuit coacts with the rectifier circuit to charge the smoothing capacitor to a required voltage. Two separate current paths are thus obtained for charging the smoothing capacitor, one through the inductor, which has been conventionally designed for a higher power factor and more sinusoidal input waveform, and the other through the ancillary charging circuit constituting the gist of this invention. The smoothing capacitor needs to be charged through the two current paths to the same voltage as through the prior art single path, so that the magnitude of the current flowing through the inductor can be lessened by an amount equal to the magnitude of the current supplied from the ancillary charging circuit. The desired objective of higher efficiency is thus accomplished as less power loss is bound to occur at the inductor. This inductor can moreover be made appreciably less in size than in the absence of the ancillary charging circuit.
The instant invention also provides for an alternative method of operation: The smoothing capacitor may be charged to a voltage higher than heretofore with the aid of the ancillary charging circuit. Charged to such a voltage, the smoothing capacitor can be prevented from the inflow of excessive current through the inductor at or adjacent the peaks of the a.c. input voltage, with consequent curtailment of the higher harmonics of the input current.
The ancillary charging circuit required by the invention should be as simple and inexpensive in construction as feasible. To this end the invention suggests use of an ancillary winding incorporated with the transformer, a standard component of this type of switching power supply, as a primary component of the ancillary charging circuit. The operational advantages gained by the ancillary charging circuit are a sufficient justification for its addition to the switching power supply, as judged from the cost performance of the resulting device.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings showing the preferred embodiments of the invention.